The effect of anticonvulsant drugs, particularly hydantoinates and barbiturates, on vitamin D stimulated calcium and phosphate transport by rat intestine is under study. By the use of vitamin D metabolities, 25- hydroxy cholecalciferol and 1-gamma hydroxycholecalciferol, it may be determined whether inhibition of intestinal transport of calcium by anticonvulsants is due to rapid inactivation of vitamin D metabolites or to a direct action on the calcium transport system of the mucosa. Feeding of diets low in phosphate to rats results in enhancement of intestinal transport of phosphate and calcium. One mechanism of such stimulation has been suggested, i.e. reduced inorganic phosphate stimulates 1-gamma hydroxylation of 25-hydroxy vitamin D. Preliminary data suggest that this is not the entire explanation and further studies are in progress to determine whether another mechanism may be operative. The interrelation of the vitamin D stimulated calcium and phosphate transport systems remains unanswered and we intend to make a more detailed examination of the phosphate transport system with regard to calcium dependency and the roll of mucosal phosphatases.